The findings stem from a review of 17 studies conducted around the world. Each assessed a possible link between blood pressure and dirty air related to common pollutants, such as vehicle exhaust, coal burning and airborne dirt or dust.

"Our results demonstrated that air pollutants had both short-term and long-term effects on [high blood pressure] risks," said study author Tao Liu. He is deputy director of the environmental health division in the Guangdong Provincial Institute of Public Health in Guangzhou, China.

In the short term, he noted, a few days of increased air pollution could lead to more emergency hospital visits due to temporary spikes in blood pressure. In the long term, those living with consistently high levels of air pollution could end up with chronically high blood pressure.

High blood pressure is a key risk factor for stroke and heart disease, the number one cause of death worldwide. And high blood pressure itself is associated with about 17 percent of fatalities globally, the study authors pointed out.

"It is urgent to take more actions to protect our environment and clean the air quality," said Liu.

For now, people—especially those with high blood pressure—should "closely focus on the air quality every day, and try to avoid outdoor activities or wear filtered masks when air quality is poor," Liu said.

According to background notes with the study, causes of high blood pressure (or "hypertension") include genes, lifestyle habits, diet, and environmental factors—probably including air pollution. Until now, evidence linking smog with high blood pressure has been controversial, Liu said.

For this project, investigators analyzed 17 studies conducted through August 2015. The studies involved roughly 328,000 people in all, about 108,000 of whom had high blood pressure.

Those investigations were conducted in Brazil, Canada, China, Denmark, Germany, Iran, Spain, Sweden, Taiwan, and the United States. Short-term pollution exposure was defined as occurring over a number of days, and long-term exposure over a number of years.

In general, the research team defined high blood pressure as a systolic blood pressure reading (the top number) above 140 mm Hg and/or a diastolic blood pressure reading in excess of 90 mm Hg. Use of blood pressure medication was also an indication of high blood pressure.

The focus was on air pollutants such as nitrogen oxide, which stems from fossil fuels that power factories and cars; sulfur dioxide, also a fossil fuel emission; ozone; carbon monoxide; and particulate matter, such as tiny dust specks, dirt, smoke, and droplets of liquid.

Short-term exposure to pollutants such as sulfur dioxide and certain types of particulate matter appeared to boost risk for high blood pressure. Also, long-term exposure to nitrogen oxide and particulate matter was linked to greater risk, the review indicated.

The findings didn't establish a cause-and-effect relationship between these elements and elevated blood pressure. Also, while it appeared that ozone and carbon monoxide were tied to higher blood pressure, these two links did not reach "statistical significance," the researchers said.

Dr. Gregg Fonarow, a professor of cardiology at the University of California, Los Angeles, agreed with Liu that the study results underscore a need for cleaner air.

"These findings suggest that strategies to effectively reduce exposure to air pollution may have cardiovascular benefits," he said.

Fonarow pointed to the American Heart Association recommendations, which in general advise avoiding outdoor activities and/or exercise when pollution levels are high.